Sodium Channelopathies of Skeletal Muscle.

The Na1.4 sodium channel is highly expressed in skeletal muscle, where it carries almost all of the inward Na current that generates the action potential, but is not present at significant levels in other tissues. Consequently, mutations of SCN4A encoding Na1.4 produce pure skeletal muscle phenotypes that now include six allelic disorders: sodium channel myotonia, paramyotonia congenita, hyperkalemic periodic paralysis, hypokalemic periodic paralysis, congenital myasthenia, and congenital myopathy with hypotonia. Mutation-specific alternations of Na1.4 function explain the mechanistic basis for the diverse phenotypes and identify opportunities for strategic intervention to modify the burden of disease.